Typically, the oldest techniques in the cellular environment are North America TDMA and GSM based on time division multiple access (TDMA). The North America TDMA and GSM based on TDMA can improve system throughput through the second- or third-generation code division multiple access (CDMA) based on the Direct Sequence Spread Spectrum (DS-SS) scheme.
Thereafter, the orthogonal frequency division multiplexing (OFDM) scheme has been adapted to WLAN communication standards such as IEEE 802.11a and IEEE 802.11g, and the orthogonal frequency division multiple access (OFDMA) scheme has been selected as technology for the next generation cellular network such as WiMAX or LTE.
In addition, the SC-FDMA acting as one modification of OFDM has robustness against the multi-path environment in the same manner as in the SC-FDMA, such that the SC-FDMA is appropriate for wideband high-speed data communication. Since the SC-FDMA requires a relatively simple complexity in the transmitter configuration, the SC-FDMA has been selected as uplink technology of the next generation cellular system.
In the broadband transmission/reception system applicable to a cellular network such as the legacy CDMA or OFMDA/SC-FDMA, a minimum unit of allocable communication resources is set to a code and a subcarrier and is then allocated to each user, such that multiple access is possible.
In addition, for access of a new user, an unused code may be allocated according to CDMA or unused subcarriers may be allocated according to OFDMA/SC-FDMA, such that user entry is allowed.
Since a minimum unit of a code or subcarrier acting as an allocable resource is decided and a data transfer rate is also fixed, a maximum number of supportable users are limited.
As a result, if there are a large number of users, it may be impossible to accommodate many users, or resource allocation between priority users may not be efficiently carried out. The number of supportable users is limited, so that system flexibility is deteriorated under a specific environment in which entrance/exiting of users on the network frequently occurs or channel variation frequently occurs.
As a simple example for increasing system flexibility to efficiently distribute communication resources between users using the conventional multiplexing and multiple access system, a CDMA system for allowing a multi-rate multi-code may be considered.
If a new user occurs in a communication network of the CDMA system, a base station (BS) confirms the presence or absence of available code dimension, such that signal design for allocating several transfer rates and several codes to each user after allowing the multi-rate and multi-code is allowed.
This signal design may be implemented through selection of a transfer rate and code of a signature waveform. However, if orthogonal codes are applied to individual users, the loss of a bandwidth occurs due to an excessive bandwidth of a chip pulse, such that spectrum efficiency is relatively deteriorated. If there is no redundant available code space, entrance of the user may not be allowed. Alternatively, if the user entrance is allowed, throughput deterioration of legacy users may unavoidably occurs.